Many industrial processes utilize salt forms of precious metals, especially silver, but also in some instances gold, mercury, palladium, platinum, rhodium, cadmium, lead and the like. For example, with regard to silver, silver nitrate solution is utilized in the meat packing industry in order to determine the sodium chloride content of meat. The silver nitrate solution precipitates the chloride ion and from that is derived a quantitative determination of the amount of salt present in meat products. After the analysis, the silver chloride remains which is contaminated with organic impurities.
In photography, outdated films, unused photographic paper, and many photographic solutions contain sources of silver. Such materials offer a source for reclaiming silver.
Other industrial source materials for reclaiming of silver include impure ores such as cerargyrite, by-product silver from sterling and silver plate manufacture obtained from electrolytes used in the electroplating process, x-ray laboratory materials from hospitals and the like.
In the past both chemical and electrolytic methods have been used to reclaim precious metals from industrial waste streams. Many of the chemical reactions result in a salt compound of the metal which can be filtered out to recover the metal. However, it is then necessary to treat the resulting salt compound in order to recover the free metal. In certain instances this has been done with chemical reducing agents. However, certain undesirable hazards have arisen with most reducing agents in that they are highly toxic and not biodegradable. Thus, while they may perform satisfactorily for reduction of a metal salt from an industrial source, they are undesirable in that they increase the toxicity, non-biodegradability, and pollution level of the factory waste materials. Thus, waste material disposal is complicated, made more expensive, and more hazardous. Often the increased problems of waste disposal far exceed any increased economic advantage of reclaiming precious metals, and thus the metals are not recovered.
Another important feature of a good silver recovery process is the condition including both purity and particle size of the reclaimed silver. If, for example, the silver has a high level of impurities, or is so small in particle size that it will not settle easily, it becomes very difficult to handle and reclaiming costs are increased.
The most commonly used alternative to chemical reclaiming of precious metals is electrolysis. However, in many instances, electrolysis does not do as complete a job of reclaiming as the use of chemical reducing agents and moreover, the initial cost of the equipment in order to complete electrolysis is quite high.
Accordingly, there is a real need for a precious metal recovery process which is substantially cheaper from the standpoint of initial investment than electrolysis recovery, and which is advantageous for industrial usage in that it does not involve increased pollution hazards or increased expense of waste disposal. In addition, there is a need for a process which utilizes only chemical constituents with those chemical constituents being very inexpensive so that the economics of previous metal reclaiming weigh heavily in favor of the reclaiming project.
This invention has as its object the fulfillment of the above described needs. In addition, other objects, which will become apparent from the detailed description which follows below, include at least the following.
A primary object of this invention is to provide a precious metal recovery process which employs a chemical reducing agent, which is inexpensive, non-toxic, does not involve increased pollution hazards, and which can suitably be used for a variety of metal reclaiming processes including recovery from waste materials in a meat packing plant as well as from photographic sources, x-ray laboratories, electroplating processes and many others.
Another object of this invention is to provide a process which minimizes the initial expense for reclaiming of precious metals.
A still further object of this invention is to provide a process which allows nearly complete recovery of all precious metals present in an industrial waste system.
A still further object of this invention is to provide a process of reclaiming precious metals, especially silver, which provides the free metal substantially free from any occluded impurities from the industrial process for which the original metal ion solution was utilized.
Another object is to provide a precious metal recovery process which uses chemicals which can be used by employees without much training in use or handling of the treating agents.
Yet another important object of this invention is to provide for silver or other precious metal recovery by a chemical precipitation process which provides the silver at high purity levels and at a particle size which will settle very easily from solution on standing for a few minutes.
The method of accomplishing these and other objects will become apparent from the detailed description of the invention which follows.